A simple sentence consists of a single independent clause with no dependent clauses, containing a subject and predicate that express a complete thought. It begins with a capital letter and ends with a period. Examples include "the baby sleeps" and "after lunch, my friend and I went to watch a movie."

1. What is a Simple sentence?
A simple sentence is a sentence consisting of one independent clause only.
There will be no dependent clause. It has a subject and predicate and can
express a complete thought. It starts with a capital letter and ends with a
period.
Examples of Simple sentence:
The baby sleeps.
After lunch, my friend and I went to watch a movie.
Reference blog:
https://typesofsentences.com